The present invention relates generally to long cavity laser systems. More specifically, the present invention relates to long cavity fiber optic laser systems, particularly frequency doubling long cavity laser systems using at least one optical fiber amplification component. Methods for generating polarization controlled laser beams using a long laser cavity formed from a rare-earth doped optical fiber are also disclosed.
At the present time, the development of new solid state media excited by high-power semiconductor lasers is an extremely active field. As a result of this work, compact systems which produce high quality beams in the near infrared have been developed. When harmonic generation techniques are utilized, compact lasers in the blue-green region of the spectrum are created. Such devices are presently of great interest because of their many applications, particularly in high density optical storage. U.S. Pat. Nos. 4,841,528, 4,887,270, 5,363,390, 5,390,210, 5,420,876, 5,450,429, and 5,479,431, which patents are incorporated herein by reference for all purposes, disclose typical frequency doubled laser systems capable of emitting a green laser light. It will be appreciated that the majority of these systems utilize short laser cavities; some of the disclosed systems utilize folded laser cavities.
A short laser cavity suffers from power fluctuations because of competition between the longitudinal modes within the laser cavity. It will be appreciated that in a short laser cavity, only a few longitudinal modes are operating at any given instant. The gain for any particular longitudinal mode is suppressed by cross saturation, both spatially and temporally, by power in other oscillating longitudinal modes. Since each longitudinal mode intersects and saturates a different volume of the gain medium, the gain for longitudinal modes that do not efficiently couple to the volume of the gain medium supporting the primary longitudinal mode can increase over time as inverted populations build up in those under-extracted volumes. In particular, the gain of such longitudinal modes can then actually exceed the gain of the then-current dominant longitudinal mode. When this happens, the frequency of the primary mode, and its frequency-doubled replica, jump to the new longitudinal mode, thereby causing a power spike and an attendant, subsequent power fluctuation. It will be appreciated that these fluctuations in the mode spectrum behave chaotically. Moreover, as mentioned above, these fluctuations in the mode spectrum are reflected in the amplitude spectrum of the frequency-doubled signal.
In contrast, a long laser cavity does not suffer from the above-described instability problems. Advantageously, a large number of modes oscillate in a long laser cavity and have sufficient overlap spatially that the gain in the laser rod is essentially homogenized to look approximately the same to all of the modes. The overall effect is that the amplitudes of the oscillating modes fluctuate less. In addition, if one longitudinal mode is overcome or quenched by another longitudinal mode, it is often only one of hundreds of lines oscillating in the long laser cavity. It will be appreciated that there is little or no effect on the total power emitted by the laser when one or more lines are quenched.
Thus, a frequency doubled laser using a medium such as Nd:YAG is highly unstable when a short laser cavity is used as the primary laser. The use of a relatively long laser cavity, i.e., long enough to permit stable operation, alleviates this instability problem.
One approach to building a frequency doubled laser using a long laser cavity was recently proposed by workers at the Semiconductor Laser Technology Branch of the Philips Laboratory (PL/LIDA), under funding from the U.S. Air Force. The proposed frequency doubled laser included a folded resonator design and used one or more discrete, end-pumped Nd:YAG rods, Different diode pump arrays were optically coupled to the rod(s) using either lenses or optical fibers.
The resonator was implemented as shown in FIG. 1A, wherein a single Nd:YAG rod (R1) is used as an active mirror in an inverse Z resonator configuration. The rod R1 was high-reflectivity (HR)-coated at 1064 nm and anti-reflection (AR)-coated at 808 nm on the rear (pump) surface and AR-coated at 1064 nm on the front surface. The resonator end mirrors (M1 and M3) were both HR-coated at 1064 nm; the end mirror in the frequency doubling leg (M3) was also HR-coated at 532 nm. The output coupler (M2) was HR-coated at 1064 nm on one surface and AR-coated at 532 nm on both surfaces.
It will be appreciated that the inverse Z resonator design allows implementation of an alternative two rod configuration as shown in FIG. 1B, wherein mirror M1 is replaced by a second Nd:YAG rod (R2). This allows pumping of each rod with a high-power laser diode, resulting in scaling to higher powers while alleviating gain media thermal loading. It will also be noted that three Nd:YAG rods (not shown) can be used in a Z-resonator when each rod is separately pumped. It should be noted that all of the inverse Z-resonator configurations employ an intracavity frequency doubling crystal (C1) made from a single piece of potassium titanyl phosphate (KTP). The KTP was mounted in a holder which allowed xyz translation, moderate xy tilt, and 360-degree rotation about the resonator axis.
As discussed briefly above, either conventional optics or optical fibers were used to couple the output of a laser diode bar or array to the Nd:YAG rod(s). In one instance, a four lens beam shaping train, i.e., three cylindrical lenses and one spherical lens, was used to focus the output of a laser diode array into the Nd:YAG rod. Later experiments used a fiber coupled diode array which was first collimated with a Melles Griot 8 mm lens and then focused into the Nd:YAG rod with a 31.7 mm spherical lens; both lenses were AR-coated at 830 mm.
It should be noted that although optical fibers were employed in the inverse Z resonator depicted in FIGS. 1A and 1B, the use of optical fibers in place of the inverse Z resonator did not occur to the system designers. From FIGS. 1A and 1B, it will also be appreciated that lengthening of the laser cavity by folding the cavity using reflectors requires careful alignment during the final stages of assembly. Moreover, the use of discrete elements dictates that a robust frequency doubled laser using a long laser cavity will require commensurate support elements, thus increasing the size and weight of the overall laser apparatus.
What is needed is a frequency doubled laser system which is compact and which provides excellent power stability. Additionally, what is needed is a frequency doubled laser system which is light in weight and which is not prone to alignment problems.